In 1934 French Patent No. 763,681 was issued to P. A. Picard. In this patent a security system detecting the distortion of an interrogation electromagnetic field by a security tag comprizing soft magnetic material (of permalloy type) was disclosed. That was the start of a new class of inventions.
Since then, for almost half a century, a great multiplicity of methods and systems related to this class has been invented and the number of such inventions is steadily growing, evidencing that the need in a truly satisfactorily performing system is still there, simply because such a system has not been invented yet.
Most of the electromagnetic security systems use the frequency-domain approach to signal processing, looking for such predetermined features of a tag signal as a certain ratio of certain harmonics (e.g. U.S. Pat. No. 4,535,323) or a phase shift of harmonics (e.g. U.S. Pat. No. 4,791,412). There are many inventions related to this approach disclosing specially synthesized magnetic materials with uniquely shaped hysteresis loops (e.g. U.S. Pat. No. 4,823,113) or uniquely constructed so called "coded" tags (e.g. U.S. Pat. No. 4,799,076). Nevertheless, these costly solutions do not provide satisfactory separation of a true tag signal from that produced by other magnetizable metal objects (e.g. shopping carts) simply because the field in the surveillance zone is not uniform and is also biased by the earth magnetic field. This often results in the tag signals and also the spurious signals from metal objects having frequency contents different from those attributed to them. This will cause either a failure to recognize the real tag or a false alarm. Periodic external noises (for example from video monitors) can also produce stable frequencies within bands open for expected tag signal frequencies.
The "frequency-domain" systems have to use a continuous transmission of the interrogation field in order to obtain sensible magnitudes of the harmonics of a tag signal. But it is possible to utilize a continuous transmission in so called "time domain" systems which are concerned with the shape of a signal rather than with the frequency content of same. U.S. Pat. No. 4,623,877 describes such a "time-domain" system with continuous transmission. This invention uses a bias provided by the earth magnetic field to the interrogation field which results in an asymmetry in the positions of tag signals with regard to periodically repeated certain points of the interrogation field. This invention claims that any other magnetic but not so easily saturated material can produce field disturbance signals at the points where the field is much stronger and therefore those signals will be more symmetric. In addition, this invention also provides periodic blanking of the signal processor at the time intervals corresponding to the amplitude levels of the field in order to ignore signals from metal objects originated in a strong field. But when placed close to one of the transmitting antennae, where the strength of the field is really high and the biasing effect of the earth magnetic field is almost negligible, the tag signals will have a good symmetry and may be ignored, whereas the metal objects will be saturated at much lower than amplitude levels of the alternating field, thus producing asymmetric signals within the time windows and therefore initiating a false alarm. The earth magnetic field is also very weak in the areas close to the equator, so this system will not be efficient if installed in many countries of Latin America or Africa or even the Middle East. As well, a periodic external noise asynchronous to the interrogation field (from video monitors, for example) can produce a sensible level of asymmetry and cause a false alarm unless long averaging is used, which makes the system slow.
The continuous way of transmission when used in conjunction with a "flat" transmitting antenna is not effective for adequate spatial distribution of the field and therefore many such systems either use antennae of complicated and cumbersome construction or just use flat antennae, sacrificing performance by accepting large dead sections within the surveillance zone.
There are only a few systems of the prior art utilizing a pulsing concept of transmission when every transmission pulse consists of several numbers of periods and there is a pause between pulses. In U.S. Pat. Nos. 4,300,183 and 4,527,152 the pulsing concept is used to change alternatively from zero to 180.degree. and vice versa the phase difference between currents in two transmitting flat coils creating together an interrogation field. This provides better coverage of the protected space when flat transmitting antennae are utilized. No other use of the pulsing transmission was disclosed in the prior art inventions, although this type of transmission, unlike the continuous one, can offer very satisfactory solutions to the false alarm problems.
The prior art systems with pulsing transmission are related to the time-domain group. For signal recognition, these systems use either a comparison of the wave shape of the distortion signal to stored samples of possible wave shapes (as was disclosed in U.S. Pat. No. 4,663,612), or (as was proposed in U.S. Pat. No. 4,527,152) decide about the presence of a tag signal by measuring the width of a pulse in the time-window, or by the use of cross correlation between a stored signal and a repeated one in order to establish how similar they are. All these methods provide neither adequate reliability of signal recognition nor protection against false alarms. It is practically very difficult to obtain a pure tag signal without altering its characteristics, considering the inevitable use of filters to suppress the main frequency of the field and its harmonics in the receiver circuitry, components of which have band limitations of their own (not to mention that in a very wide-banded system the noise level can swallow the signal completely). Therefore, both original tag signals (even if uniquely shaped as was suggested in U.S. Pat. No. 4,686,154) and spikes of noise are reshaped in the receivers, often acquiring shapes which are similar to those stored as the samples they are to be compared with. The method of pulse width measurement can cause severe false alarming in a noisy environment, and cross-correlation methods are totally helpless against a succession of identical spurious signals originated either by metal objects in the interrogation field or induced by external periodic fields from, for example, horizontal deflection units of video monitors.